In order to apply a high molecular solid electrolyte to electrochemical devices such as a battery, the electrolyte needs to exhibit not only good ionic conductivity to reduce its resistance but also excellent film-forming properties and high film strength to prevent internal shortcircuiting. Further, the starting materials of the high molecular solid electrolyte need to be easily prepared. However, a high molecular solid electrolyte satisfying all of these requirements has never been developed.
For example, vinyl polymers having polyethylene oxide groups in side chains (hereinafter referred to as "PEO") as disclosed in D. J. Banister et al., "Polymer", 25, 1600, 1984, D. F. Shriver et al., "Journal of American Chemical Society", 106, 6854, 1984. Materials comprising low molecular PEO incorporated in polysiloxane are disclosed in Watanabe et al., "Journal of Power Sources", 20, 327, 1987. These high molecular solid electrolytes exhibit excellent film-forming properties and high strength. However, all these high molecular solid electrolytes also exhibit a low ionic conductivity and thus cannot be put into practical use.
In order to enhance the ionic conductivity of such materials at room temperature, a material comprising a liquid ionic conductor retained in a high molecular solid electrolyte has been studied. For example, JP-B-61-23947 (The term "JP-B" as used herein means an "examined Japanese patent publication") discloses a material comprising 90% by weight or more of an organic electrolyte having a relative dielectric constant of 10 or more incorporated in a high molecular solid electrolyte having a relative dielectric constant of 4 or more. However, the ionic conductivity of such a material is not sufficient. Further, JP-A-63-135477 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a material comprising a low molecular PEO retained in a crosslinked high molecular matrix containing PEO group. Such a material also exhibits insufficient ionic conductivity.
Further examples of the enhancement of the ionic conductivity of such a material at room temperature are materials comprising a high molecular matrix having a polar group in side chains impregnated with an aprotic polar solvent such as propylene carbonate (hereinafter referred to as "PC") as disclosed in JP-B-57-9671, JP-A-63-239779 (U.S. Pat. No. 4,822,701), U.S. Pat. No. 4,830,939, and EP026084. However, these materials must retain a large amount of such a solvent to exhibit an enhanced ionic conductivity. Thus, these materials exhibit a remarkably poor film strength and hence cannot be put into practical use.
One known approach for the enhancement of film strength uses a material comprising a porous membrane such as nonwoven fabric impregnated with a high molecular solid electrolyte. For example, JP-A-60-195878 discloses a material comprising a PC solution of polyvinylidene fluoride coated on a nonwoven fabric. However, even such a material cannot attain a sufficient ionic conductivity. Further, JP-A-61-219469, JP-A-63-40270, and JP-A-63-102104 disclose similar approaches. However, none of these approaches can provide satisfactory ionic conductivity and film strength altogether.
Known methods for the preparation of these high molecular solid electrolytes include a method which comprises heating a monomer in the presence or absence of a solvent capable of dissolving the monomer therein (as disclosed in JP-A-62-50263, JP-A-1-241764), polymerization by ultraviolet rays (as disclosed in JP-A-63-135477), and polymerization by electron rays (as disclosed in JP-A-2-602). All these methods can provide an excellent ionic conductivity but a poor film strength.
Further, an approach which comprises spray-drying a latex of polyvinyl chloride, and then heat-melting the material to form a film which is then used as a separator is disclosed in Romanian Patent 96,422. This separator provides a sufficient film strength in an aqueous solution system battery but is disadvantageous in that it is subject to dissolution or swelling and provides high resistance in a nonaqueous electrolyte system.
As mentioned above, these high molecular solid electrolytes cannot provide a solution to all the foregoing problems such as remarkably low ionic conductivity at room temperature and poor film strength. It has thus been desired to provide a solid electrolyte providing a solution to all these problems.